FAQ
TL;DR: For a 10 kW, 400 V star‑connected heater, each phase draws 14.5 A; "14.5 A from each phase." Choose 20 A per‑phase fuses. [Elektroda, Pyzaczek, post #17935702]
Why it matters: For electricians, techs, and DIYers who need to size fuses and understand phase currents on three‑phase heaters fast.
Quick Facts
- In star on 400 V systems, each branch sees 230 V; current I = P/230 (≈14.5 A at 3.33 kW). [Elektroda, kortyleski, post #17938421]
- Practical fuse pick: 20 A per phase for a 14.5 A branch current. [Elektroda, Pyzaczek, post #17935702]
- Cold elements draw higher inrush; resistance is lower at switch‑on. [Elektroda, nuszek, post #17935861]
- Connect N to the star point so two heaters keep working if one fails. [Elektroda, kortyleski, post #17935555]
- For resistive heaters, calculate with cos φ = 1 and phase current If. [Elektroda, retrofood, post #17935781]
How do I calculate phase current for a 10 kW three‑phase heater on 400 V?
For a star heater with three equal elements, use per‑branch values. Each element is about 3.33 kW at 230 V. The current per phase equals branch current for star. I_phase = P_branch / 230 V ≈ 14.5 A. That matches the thread’s confirmed result. [Elektroda, Pyzaczek, post #17935702]
Is the 25 A from 10000/400 the total line current?
That 25 A comes from a single‑phase formula, so it misleads here. In a 400/230 V star, calculate using 230 V per branch. Each phase then carries about 14.5 A. Do not add phase currents together. [Elektroda, kortyleski, post #17938421]
What fuse size should I use per phase for this heater?
The thread recommends 20 A per‑phase protection for a 14.5 A branch current. This gives working margin without nuisance trips. Confirm device curve and ambient limits with your protection hardware’s datasheet. [Elektroda, Pyzaczek, post #17935702]
Why should I connect the neutral to the star point?
Connecting N to the star point stabilizes voltages during a failure. If one element opens, the other two still see 230 V. Without N, unbalance can over‑ or under‑voltage the remaining elements. It is a simple reliability gain. [Elektroda, kortyleski, post #17935555]
How do star and delta wiring change phase and branch currents?
In star, the line current equals the current of one heater element. In delta, the line current differs from the element current. As one expert put it, “In a star, phase current = current of 1 heater.” Use correct terms to avoid errors. [Elektroda, retrofood, post #17938886]
How can I verify whether my heater is wired in star or delta?
Kill power and open the terminal box. Look for three element ends tied together; that is the star point. Measure one element’s resistance to confirm continuity and estimate current. Then match the nameplate and compute branch current at 230 V.
- Isolate power and lock out.
- Open the box; check for a tied star point.
- Measure element resistance; compute I = 230 V / R. [Elektroda, Pyzaczek, post #17938796]
Why does the nameplate say 3×400 V if each element is 230 V?
Manufacturers often mark the supply system rather than element voltage. In a 400/230 V system, star‑connected elements each see 230 V. The OP confirmed this: “the heater is powered by 3×230V.” The 3×400 V label denotes the three‑phase supply. [Elektroda, wiesko42, post #17946442]
What current does the whole 10 kW heater draw from the mains?
Expect about 14.5 A in each of the three phases. Do not multiply by three for total current. Size upstream protection based on the per‑phase current and device characteristics. [Elektroda, Pyzaczek, post #17935702]
Is there a calculator I can use, and what settings?
Yes. Use a three‑phase power–current calculator and set cos φ = 1. Read If as the current of one phase. The PRS calculator linked in the thread is suitable. “Use cos φ = 1, If – current of one phase.” [Elektroda, retrofood, post #17935781]
What happens at switch‑on with cold heater elements?
Cold elements have lower resistance, so inrush current exceeds the steady 14.5 A. Select protection with headroom to ride through this transient. This avoids nuisance tripping at startup. [Elektroda, nuszek, post #17935861]
What resistance did users measure on a single heater element?
One measurement reported approximately 17.5 Ω for a single element. That reading came from a heater confirmed as star‑connected. Use your multimeter to verify your unit. Differences reflect tolerance and temperature during measurement. [Elektroda, wiesko42, post #17938995]
If one element burns out, will the other two still work?
With the neutral tied to the star point, the other two keep normal operation. Without neutral, voltages can shift and stress the remaining elements. Connecting N is a simple reliability improvement. [Elektroda, kortyleski, post #17935555]
Is phase current about 8 A if the heater is delta‑connected?
No. That figure is incorrect for a 10 kW three‑phase heater. In delta, the relation between line and element currents differs from star. Use correct definitions before calculating currents. [Elektroda, retrofood, post #17938886]
